Developments of a transmission system and an optical fiber that use Wavelength Division Multiplexing (WDM) have been actively advanced with the objective of increasing the data transmission rate in backbone and/or long-distance systems. Characteristics such as suppression of the nonlinear and control of the chromatic dispersion effect have been demanded for optical fibers for WDM transmission. In recent years, fibers in which the dispersion slope is decreased for a system called metro (metropolitan area network) with a span of about several kilometers, or fibers that suffer virtually no loss increase due to the presence of a hydroxyl group (OH), are proposed.
On the other hand, when introduction of fibers to offices and homes (Fiber To The Home; FTTH) is taken into consideration, characteristics different from those for the above-described optical fibers for transmission are required. In the case of wiring the fibers in the building or the house, there is the possibility that a very small bending around a radius such as 15 mm or 10 mm occurs. Furthermore, when the extra length of the fiber is stored, it is very important that loss increase does not occur, even if the fiber is wound around a small radius. That is, it is a very important characteristic for the optical fiber for FTTH to be insensitive to a small-radius bend. The connectivity with optical fibers (many of which are SMFs for transmission at a normal wavelength, i.e., 1300 nm) used from the base station to the building or to the house is also an important point. From such viewpoints, many reports on and patent application of optical fibers with reduced bending loss have been made (for example, refer to Patent Documents 1 to 4 and Non-Patent Documents 1 to 5).
The following documents describe the related art for the current invention:    Patent Document 1: U.S. Patent Application Publication No. 2004/0213531    Patent Document 2: PCT International Publication No. WO 01/27667 pamphlet    Patent Document 3: Japanese Unexamined Patent Application, First Publication No. 2004-133373    Patent Document 4: Japanese Patent No. 2618400    Non-Patent Document 1: Ikeda et al., “Low Bending Loss Optical Fiber with Reduced Splice Loss,” Technical Report of The Institute of Electronics, Information and Communication Engineers (IEICE), 103, 255, OCS 200343 (2003)    Non-Patent Document 2: Sato et al., “Optical Fiber Conforming to Small Bending Radius for Optical Access,” Collected Papers of Lectures of IEICE Society Conference, B-10-30 (2003)    Non-Patent Document 3: S. Matsuo, et al., “Bend-insensitive and Low-splice-loss optical fiber for indoor wiring in FTTH”, Technical Digest of OFC 2004, ThI3 (2004)    Non-Patent Document 4: Ikeda et al., “Low Bending Loss Optical Fiber with Reduced Splice Loss,” Collected Papers of IEICE General Meeting 2004, B-10-1 (2004)    Non-Patent Document 5: I. Sakabe, et al., “Enhanced Bending Loss Insensitive Fiber and New Cables for CWDM Access Network,” Proceedings of the 53rd IWCS, pp. 112-118 (2004)
In the current FTTH system, a Passive Optical Network (PON) that employs SMFs for transmission in the 1300-nm wavelength band is widely used.
However, these related art optical fibers have an allowable bending radius of about 30 mm. In the wiring of the fibers, close attention has been required to eliminate extra bending.
Recently, SMFs have been commercialized that have allowable bending radii down to 15 mm by reducing the mode field diameter (hereinafter, referred to as MFD) while maintaining chromatic dispersion characteristics that comply with ITU-T G.652 (hereinafter, referred to as G.652), the international standard for the SMF for the 1300-nm wavelength band. However, such optical fibers have a problem in that the bending loss suddenly increases with a bending radius of 15 mm or less. FIG. 1 is a graph exemplifying a bending radius dependence of the bending loss of an optical fiber with an allowable bending radius of 15 mm. As shown in FIG. 1, the optical fiber with the related art allowable bending radius of 15 mm suffers a sudden increase in bending loss when the bending radius falls below 10 mm.
For a wiring use in buildings and homes, there are cases where a bending radius of 15 mm or less is needed. The optical fibers proposed in the above-mentioned Patent Documents 1 to 3 and Non-Patent Documents 1 to 5 are assumed to be used in an environment in which a bending around a 15-mm radius or less may be applied. An optical fiber with enhanced bending characteristics generally has a longer zero-dispersion wavelength, and thus has a larger absolute value of the chromatic dispersion in the 1300-nm wavelength band, compared with the normal SMF. For example, Patent Document 1 shows an example in which the optical fiber with low bending loss disclosed therein has a chromatic dispersion of −4.6 to −10.7 ps/nm/km in the 1300-nm wavelength band. The absolute value of the chromatic dispersion of this related art optical fiber with low bending loss is large, compared with the fact that the chromatic dispersion in the 1300-nm band under G.652 is in the range of 0 to −2.2 ps/nm/km, when calculated by the definition of the zero-dispersion wavelength and the slope under G.652. However, the chromatic dispersion on this level has rarely posed a problem over a distance on the order of several tens of meters, such as in indoor wiring.
On the other hand, there is a need also for optical fibers for transmission lines to be resistant to bending loss when their handling in a cable or a closure box is taken into consideration. However, there are cases where the chromatic dispersion value of the low bending loss optical fiber shown in Patent Document 1 poses a problem when the fiber is used in the PON system. Under ITU-T G.983 or similar rules, in the PON system used for the FTTH service, the 1500-nm wavelength band is used for the transmission from the base station to the user, and the 1300-nm wavelength band is used for the transmission from the user to the base station. As for the light source for the 1300-nm wavelength band, an inexpensive Fabry-perot laser (hereinafter, referred to as FP laser) is widely used. Because the FP laser is based on multimode oscillation, the characteristics thereof are greatly affected by the chromatic dispersion value of the optical fiber that works as the transmission line. Current transmission apparatuses are designed with the chromatic dispersion characteristics of G.652 in mind. Therefore, there are cases where a large-absolute-valued chromatic dispersion value that the related art low bending loss fibers have is not preferable, because it may cause a communication failure.